1. Field of the Invention
The present invention relates to a synchronization establishing device that establishes synchronization in a receiver, and a method of establishing synchronization. The present invention also relates to the receiver.
2. Description of the Related Art
In a receiver that employs a CDMA (Code Division Multiple Access) system, the top of a slot (one group of data) contained in a reception signal is searched for by detecting the strength of a synchronization signal. This is called “cell search”. A pattern contained in the slot is then detected to search for the top of a frame made up of 15 slots. This is called “second code search”. The reception signal, of which cycles have been roughly grasped through the above searches, is more precisely detected so as to maintain synchronization. This is called “path search”. Thus, the data contained in the reception signal can be received.
In the second code search, a spreading code is conventionally used. An example of such a spreading code is shown in FIG. 1. As shown in FIG. 1, the spreading code is an (N×M)-bit spreading code generated by multiplying a common code Z that comprises N codes a(0) to a(N−1) by an intrinsic code that comprises M code bn(0) to bn (M−1) (a bit string). The N code will be also referred to as “N chips”, and the M code will be also referred to as “M chips”. The (N×M)-bit spreading code will be referred to as “(N×M) chips”.
FIG. 2 shows the structure of a conventional synchronization establishing device used in the second code search. As shown in FIG. 2, this synchronization establishing device comprises a random access memory (RAM) 101, a correlator 102 for correlating a common code, and correlators 103a to 103d for correlating intrinsic codes. The correlator 102 is connected to the RAM 101, and the correlators 103a to 103d are connected to the correlator 102 in parallel with each other.
In the synchronization establishing device having the above structure, the RAM 101 temporarily stores a reception signal, and the correlator 102 reverse-diffuses common code parts in the reception signal supplied from the RAM 101 in accordance with a supplied common code. The correlator 103a reverse-diffuses the reception signal supplied from the correlator 102 in accordance with an intrinsic code 0, thereby generating a code-0 correlation value. Likewise, the correlators 103b to 103d each reverse-diffuse the reception signal supplied from the correlator 102 in accordance with one of intrinsic codes 1 to n, thereby generating the correlation value of each of the codes 1 to n.
FIGS. 3A and 3B illustrate the operation performed by the conventional synchronization establishing device shown in FIG. 2. As shown in FIG. 3A, the correlator 102 for the common code carries out despreading (integration) in the N process cycles at the timing indicated by the arrows, in accordance with the common code made up of the N chips. As shown in FIG. 3B, the correlators 103a to 103d for intrinsic codes carry out integration M times at the timing shown by the arrows, in accordance with the intrinsic code made up of the M chips. Accordingly, the conventional synchronization establishing device requires the N×M process cycles for the despreading process through the second code search. As for a receiver employing a CDMA system, however, there is a demand for high-speed synchronization establishment through the detection of code phases of reception signals.